1. Field of the Invention
The present invention relates to a power amplifier, a power amplifying method, and a power amplifier with predistortion compensation. The present invention specifically concerns a power amplifier, a power amplifying method, and a power amplifier with predistortion compensation that are used mainly at radio communication base stations of mobile phones.
2. Related Art of the Invention
In recent years, there has been a demand in base station transmitters, for high-efficiency power amplifiers with high linearity. These amplifiers are required to amplify a number of signal channels in a collective manner. In order to increase the linearity of power amplifiers, for example, it is necessary to adopt distortion compensating circuits of a predistortion system or the like.
FIG. 29 is a block diagram showing a conventional power amplifier with predistortion compensation. Reference numeral 601 denotes an input terminal, reference numeral 602 denotes an output terminal, reference numeral 603 denotes a power divider, reference numeral 604 denotes a delay circuit, reference numeral 605 denotes a distortion generating circuit, reference numeral 606 denotes a variable attenuator, reference numeral 607 denotes a variable phase shifter, reference numeral 608 denotes a power combiner, reference numeral 609 denotes an amplifier, reference numeral 610 denotes a power divider, and reference numeral 611 denotes a control circuit.
In the power amplifier configured with predistortion compensation, a modulated carrier signal inputted to input terminal 601 is divided into two in the power divider 603. In response to one of the divided modulated carrier signals, the distortion generating circuit 605 generates a distorted signal. The distorted signal is adjusted in amplitude and phase in the variable attenuator 606 and the variable phase shifter 607. The resultant signal is fed to the power combiner 608. The other modulated carrier signal having been divided in the power divider 603 is delayed in the delay circuit 604, and then, the signal is inputted into the power combiner 608. The power combiner 608 combines the distorted signal and the modulated carrier signal and inputs the combined signal to the amplifier 609. The amplifier 609 amplifies the inputted signal and outputs the signal from the output terminal 602.
The power divider 610 is provided between the amplifier 609 and the output terminal 602. An output signal from the amplifier 609 is partially divided in the power divider 610 and is fed to the control circuit 611. The control circuit 611 controls the variable attenuator 606 and the variable phase shifter 607 so that the distorted signal inputted to the power combiner 608 is equal in amplitude to an adjacent channel leakage distortion, which is generated when the amplifier 609 amplifies a modulated carrier signal (hereinafter, simply referred to as “distortion” or “ACP”), in opposite phase.
As described above, in the power amplifier with predistortion compensation shown in FIG. 29, a distorted signal is generated which is equal in amplitude in opposite phase to distortion expected to be generated when the amplifier 609 amplifies a modulated carrier signal, and the signal is added to a modulated carrier signal to be inputted to the amplifier 609 (that is, a distortion component equal in amplitude to generated distortion in opposite phase is inputted to the amplifier), so that distortion generated in the amplifier 609 is reduced. Such a circuit configuration is disclosed, for example, Japanese Patent Laid-Open No. 2000-261252. The disclosure of the above reference is incorporated herein by reference in its entirety.
However, in the conventional power amplifier with predistortion compensation shown in FIG. 29, when a level difference is large between an adjacent channel leakage distortion component generated on the low-frequency side (hereinafter, referred to as ACPL) and an adjacent channel leakage distortion component generated on the high-frequency side (hereinafter, referred to as ACPU) in the amplifier 609, ACPL and ACPU generated in the distortion generating circuit 605 are both difficult to equalize in amplitude in opposite phase to ACPL and ACPU generated in the amplifier 609. Consequently, it is not possible to have a large suppressing amount of distortion on both of ACPL and ACPU.
In view of the above problem, the present invention has as its object the provision of a power amplifying method or a power amplifier whereby a level difference can be reduced between ACPL and ACPU generated in the power amplifier.